Charged particle therapy is used to treat certain conditions (e.g., cancer) in patients using focused, collimated or other spatially limited energetic particle beams. The principle generally relies on the controlled and localized deposition of sufficient energy in a treatment volume. The treatment volume may be an arbitrary three-dimensional volume (e.g., a cancer tumor) within the patient's body. In some instances, ionizing radiation is used to physically overcome the diseased tissue's survival thresholds and thereby destroy the diseased tissue.
In all such therapy procedures it is important to control the amount and location of the applied therapy beams and fields applied to a patient's body to avoid or minimize harm to healthy tissues and organs in the vicinity of the diseased volume. Surgical planning routines, sometimes employing medical imaging to guide the therapy procedure, are used to define the treatment volume and to prescribe the application of the therapy to the treatment volume. Time-dependent modeling, monitoring and other controls are employed to safely carry out proton therapy and similar treatments because the energy beams used in the treatments can accidentally injure the patients if applied incorrectly.
Pencil beam proton and other light ion therapy is used because of its ability to deliver dose to the patient with greatly improved spatial resolution and accuracy. It employs relatively narrow cross-sectional beams of protons, which can be on the order of a few millimeters in diameter. The advantages of the method require that the proton beam is positioned with a high degree of precision.
FIG. 1 illustrates a basic light ion therapy system such as a pencil beam proton therapy system (PBS) 10. Current proton therapy systems 10 include a proton beam source 100, which can generate a directed beam of ionizing radiation 102 at a desired energy level (typically 30 to 250 MeV). The beam 101 is transported from the source to the scanning system 120 (“Nozzle”) and deflected as needed using one or more primary bending electromagnets 112, fine trim electromagnets 114 or other components in beamline 110, as well as scanner deflectors 122 in scan nozzle 120. An ion chamber (sometimes “IC”) 124 is disposed before the target of the beam. The target is supposed to be at a location in a patient, but it is characterized for control purposes by its projection onto the nominal “isocenter” plane 105. The resulting beam reaching the patient may be deflected, intentionally or unintentionally, scanned or otherwise controlled by factors causing its beam angle and position in three dimensional space to be altered over time. Those practiced in the art have also recognized that the beam tends to deviate from its commanded position between treatment sessions and during treatments according to unwanted variation in magnetic fields and other factors affecting the beam's spatial positioning. These variations are generally imposed onto a series of commanded positions, and can potentially adversely affect the continued accuracy and effectiveness of the treatment by negatively affecting healthy organs in the vicinity of the diseased treatment volume.
If a beam has moved away from its desired trajectory by a clinically unacceptable amount, the beam must be stopped and therapeutic treatment halted. Appropriate adjustments can be made to the system to correct the offset of the beam based on the last measured position error.
Such a process of error correction increases the time for treatment, leading to increased expense. It also requires operator intervention, with an associated possibility of operator error. Extended treatment time may also introduce errors due to patient movement and associated deviations between the actual patient position and diagnostic imaging data. However, error correction may not be possible if the error is too large or if the error cannot be corrected quickly, in which case patient irradiation is abandoned.
Moreover, it is generally not acceptable to retune the beam if such retuning includes the beam reaching the patient as this could compromise the intended dose distribution for the patient in a way that is not correctable.
What is needed is an apparatus and method for maintaining beam alignment without impacting patient treatment.